The communications industry has continually struggled to provide communication equipment at affordable prices. Such communication equipment can include by way of example, mobile and wireless devices such as radios, cellular telephones, cordless phones and the like. Such devices, however, typically operate within narrow frequency ranges with little tolerance for error.
In order to operate at the desired frequencies, such devices often rely on oscillator circuits. Oscillator circuits vibrate or resonate at desired frequencies. A variety of oscillator circuits exist, some of which use transistor networks, feedback circuits, piezoelectric substances and the like.
Conventional systems typically tune an oscillator by varying the amount of capacitance coupled to the oscillator with what are called variable capacitors. In general, a variable capacitor allows one to select a desired capacitive value from a range of capacitive values. For example, adjusting the capacitive value in a variable capacitor alters the amount of capacitance coupled to the oscillator. As a result, the capacitive value modifies the operating frequency of the oscillator.
In many applications, it is desirable to provide a variable capacitor which can be finely tuned. Fine tuning allows a circuit designer to precisely select the value of the variable capacitor with relatively small incremental adjustments. In many conventional variable capacitors, however, the incremental adjustments may not be linear, that is, an incremental change may not be proportionately similar to the previous or following incremental change.
In addition to fine tuned precision, circuit designers also desire the ability to select from a large range of capacitance values. A variable capacitor which provides a relatively wide range of values is said to have a relatively large dynamic range. The large dynamic range is typically achieved by combining capacitors. The ratio of the largest capacitor to the smallest capacitor is often called the capacitance spread. A high-spread capacitance array combines large capacitance values with small capacitance values. On the other hand, in a low-spread capacitance array, the largest capacitor value is closer in value to the smallest capacitor value.
Other applications often need variable capacitors which are stable. That is, once selected, the capacitance value remains relatively constant. In many conventional variable capacitors, however, the value of the selected capacitance can vary due to changes in temperature, operating parameters and external influences.
One common type of variable capacitor used in the communications industry is a varactor diode. In order to improve the performance of varactor diodes, the communications industry has, in some cases, added circuitry to more accurately control the varactor diodes. Unfortunately, varactor diodes are non-linear, thus fine tuning can be difficult to implement. Furthermore, the dynamic range of a varactor diode is dependent on the power supply voltage. The larger the power supply voltage, the larger the dynamic range. Thus, to provide a large dynamic range requires higher voltages.
In addition, conventional varactor diodes and their control circuits are often more sensitive to power fluctuations, can dissipate more power and can add additional weight, can increase manufacturing costs and can consume additional space. As can be appreciated, designers of electronic circuits and particularly designers of hand-held communications devices, desire to improve operating performance, decrease power dissipation, conserve space and reduce weight.